Well the main organisation has been lately a bit powerless, but it still serves a good purpose. Mainly due to the fact that it's very easy to ignore the UN, due to lack of an enforcer. (but in a way this lack of an enforcer is okay in the current role of the UN)
On the other hand it's suborganisations like UNICEF do a very important job for the world and the suborganisations compensate for the UN.